Electric Current
Electric current is the rate at which net charge crosses a cross-sectional area. Conventional current follows the direction positive charge would move, even though electrons in ordinary metal wires move in the opposite direction.
Electric Current: quick answer
Electric current is the rate at which net charge crosses a cross-sectional area. Conventional current follows the direction positive charge would move, even though electrons in ordinary metal wires move in the opposite direction. Current is a scalar rate with an assigned circuit direction, not a vector in space. The most important distinction is between net charge transport and microscopic thermal motion: a wire can contain rapidly moving carriers even when the net current is zero.
Keep the course model at charge-flow rate, current direction, potential difference, and charge-carrier motion. Do not require microscopic current-density or drift-speed equations that are not needed for this topic.
What you should be able to explain and do
Use these as closed-note retrieval prompts before looking at worked examples.
Use current as charge-per-time through a cross section.
Distinguish conventional current direction from electron motion in metallic conductors.
Explain why zero current does not mean every carrier is motionless.
Use charge conservation to compare current at different positions in a single steady branch.
Key terms you need for this lesson
Definitions stay beside the circuit model so unfamiliar terminology does not break the reasoning chain.
electric current
electric current is the rate at which net electric charge crosses a chosen cross-sectional area.
conventional current
conventional current is the direction assigned to current as the direction positive charge would move.
charge carrier
charge carrier is a mobile charged particle or other mobile charge-bearing entity that transports charge through a medium.
steady current
steady current is a current that does not change with time at the location being considered.
cross section
cross section is an imagined surface cutting across a conductor through which charge flow is counted.
ampere
ampere is the SI unit of current, equal to one coulomb per second.
Know what each relationship means and when it applies
The relationship, variables, topology condition, and physical anchor belong together.
| Relationship / model | Variables & units | When to use it | Physical anchor |
|---|---|---|---|
I = ΔQ/Δt | I current (A = C/s); ΔQ net charge crossing a cross section (C); Δt time interval (s) | average current through a chosen cross section | 6 C crossing in 3 s corresponds to 2 A |
ΔQ = IΔt | ΔQ charge (C); I current (A); Δt time (s) | constant current over the interval | 0.50 A for 8 s transfers 4 C |
I = 0 ⇒ no net carrier motion | I current; carrier thermal motion may still be nonzero | distinguishing zero current from zero microscopic particle speed | an open circuit can contain moving electrons microscopically while having zero net charge flow |
What a strong circuit diagram or graph must show
Translate the same physical model among topology, arrows, graphs, equations, and energy/charge reasoning.
Wire cross section
count net charge crossing an imaginary cut per unit time.
Direction arrow
conventional current follows hypothetical positive-charge motion; electron drift in a metal is opposite.
Charge-versus-time graph
slope gives current over the interval.
Single-branch continuity
in steady state the same current passes successive cross sections of one unbranched path.
Change one circuit condition and inspect what responds
Make a prediction before moving a control, then explain the output using topology, conservation, or the governing relationship.
Two examples with the model choice shown
The solution exposes the circuit reasoning pathway rather than only the arithmetic.
Charge-through-wire calculation
Situation: A total of 18 C crosses a wire cross section in 6.0 s.
Reasoning: Use I=ΔQ/Δt because the problem gives transferred charge and elapsed time.
I=18/6.0=3.0 A.
Electron direction
Situation: A diagram shows conventional current clockwise in a copper loop.
Reasoning: Copper conduction is due mainly to electrons, whose negative charge means their net drift direction is opposite conventional current.
The electrons drift counterclockwise even though the conventional current arrow is clockwise.
Recognize the physics before reaching for an equation
Use the cue to choose the model, connect it to nearby ideas, then reject any method whose assumptions do not match the situation.
What should trigger Topic 11.1?
Use current when the question tracks how much charge crosses a location per unit time or compares conventional-current directions.
How this idea fits the course
Current is the flow variable used by every circuit topic that follows, especially resistance in 11.3 and junction conservation in 11.7.
Scope check before solving
Keep the course model at charge-flow rate, current direction, potential difference, and charge-carrier motion. Do not require microscopic current-density or drift-speed equations that are not needed for this topic.
Turn this topic into measurable evidence
Choose variables and meters that test the intended relationship without accidentally changing the circuit model.
Investigation target
Measure current at several positions in a series circuit to test charge-flow continuity.
Reasoning errors to eliminate
Repair the circuit model before doing more arithmetic.
Six things you should be able to do before moving on
Use these as closed-note transfer prompts.
- I can identify the circuit topology before applying series/parallel formulas.
- I can define every electrical quantity and give its SI unit.
- I can distinguish charge conservation at junctions from energy conservation around loops.
- I can interpret current, potential, resistance, and power from a graph or schematic.
- I can place ideal meters correctly and predict qualitative loading effects of nonideal meters.
- I can design or critique a circuit experiment and justify a claim from measured evidence.
Practice is kept on its dedicated pages
Finish the lesson first. Then open the separate topic bank when you want exam-style practice without replacing the teaching content above.
42 multiple-choice questions
Use the exact-topic MCQ bank to test model selection, representations, calculations and boundaries after learning the concept.
Open 42 Topic MCQs4 multipart free-response tasks
Use the exact-topic FRQ set for mathematical, representation, experimental and qualitative/quantitative reasoning.
Open 4 Topic FRQsTopic 11.1 FAQs
What does 1 ampere mean?
One ampere means one coulomb of net charge crosses a chosen cross section each second.
Is current a vector?
No. Current is a scalar rate, although a direction along a circuit path is assigned to it.
Which way do electrons move in a metal?
Their net drift is opposite the conventional-current direction.
Can electrons still move if current is zero?
Yes. Individual carriers can have microscopic motion while the net transport through a cross section is zero.
Why can current be the same along one series branch?
In steady state, charge cannot continually accumulate between successive cross sections, so the charge-flow rate remains the same.
What drives charge through a circuit?
A potential difference establishes the electrical conditions that produce net carrier motion through conducting paths.